1. Field of the Invention
The present invention relates to a vulcanizable rubber composition having an excellent adhesive property when vulcanized in contact with metals.
2. Description of the Prior Art
In the production of metal reinforced rubber articles, such as steel cord tire, belt or hose, and metal-rubber composite articles, such as rubber vibration insulator, resilient mounting or rubber crawler, it has been earnestly desired that the adhesion between rubber and metal should be secure as strong as possible.
The efficiency and the like of said rubber articles depend on the degree of adhesion, so that the adhesion between rubber and metal is recognized fundamentally as an important problem to the production of said metal reinforced rubber articles or metal-rubber composites.
Recently, steel cord tires have been widely used as high speed durable tires and the problem described above has become important.
Conventional methods for adhesion between rubber and metal are classified in two groups. The one method involves bonding a vulcanized rubber composition to metal materials by using some kinds of adhesives and the other method involves pressing an unvulcanized rubber composition on metal surfaces under thermal condition in order to complete both of the adhesion and the vulcanization at the same time.
The latter method has been widely applied because it is simple and useful and has an advantage of not requiring any complicated steps, such as adhesive coating step and drying step. Further, in said method, various improvements have been made for practical usage.
As an embodiment, in the rubber article to endure the repeated dynamic strain, such as steel cord tire, the surface of a metallic material, such as cord or beed wire, is plated with copper-alloy, such as brass or bronze, and thereafter an unvulcanized rubber composition containing sulfur is pressed under thermal condition to form a chemical bond between the rubber composition and copper-alloy through sulfur.
However, the adhesion strength is not satisfied and is not necessarily sufficient for the usage described above.
Another known method involves incorporating an organic acid salt of cobalt, such as cobalt naphthenate, cobalt stearate and the like, into the unvulcanized rubber composition to improve the adhesion strength. However, according to this conventional method, the rubber composition incorporated with a cobalt compound is deteriorated oxidatively in unvulcanized state or even in vulcanized state.
Especially, in the condition of unvulcanized state, the oxidative deterioration of the rubber composition is remarkable and for example, the rubber stock, such as natural rubber or polyisoprene, becomes soft and sticky on its surface within one or two weeks.
Accordingly, the unvulcanized rubber composition containing an organic acid salt of cobalt is limited in the storage period until the vulcanization, so that it is impossible to send such a rubber composition to the overseas factories.